The present invention relates generally to data transmission in mobile communication systems and more specifically to methods for managing HARQ process numbers for downlink carrier aggregation.
As used herein, the terms “user agent” and “UA” can refer to wireless devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices that have telecommunications capabilities. In some embodiments, a UA may refer to a mobile, wireless device. The term “UA” may also refer to devices that have similar capabilities but that are not transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an enhanced node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). additional characteristics to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the term “access device” will refer to any component, such as a traditional base station or an LTE or LTE-A access device, that can provide a UA with access to other components in a telecommunications system.
In mobile communication systems such as the enhanced universal terrestrial radio access network (E-UTRAN), an access device provides radio access to one or more UAs. To facilitate radio access, the access device and UA establish several communication channels including, among others, a Physical Down link Control CHannel (PDCCH), a Physical Downlink Shared CHannel (PDSCH), Physical Uplink Shared CHannel (PUSCH) and a Physical Hybrid automatic repeat request Indicator CHannel (PHICH). The access device comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UAs communicating to the access device. The functions of the scheduler include, among others, dividing the available air interface capacity between UAs, deciding the transport channel to be used for each UA's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for PDSCH and PUSCH data transmissions, and sends scheduling information to the UAs via the PDCCH.
Several different data control information (DCI) message formats are used by LTE access devices to communicate data packet resource assignments to UAs via the PDCCH. For uplink resource grants, a DCI format 0 is employed which includes, among other information, a new data indicator (NDI) which, as the label implies, indicates if the resource grant is for new data or data that is to be retransmitted. Other DCI formats are used to schedule downlink transmissions. UAs refer to the scheduling/resource allocation information for the timing and the data rate of uplink and downlink transmissions and transmit or receive data packets accordingly.
Hybrid Automatic Repeat reQuest (HARQ) is a scheme for re-transmitting a traffic data packet to compensate for an incorrectly received traffic packet. A HARQ scheme is used both in uplink and downlink in LTE systems. In the case of uplink transmissions, for each uplink packet received, the PHICH after a cyclic redundancy check (CRC) performed by the access device indicates a successful decoding. If the CRC indicates a packet is not received correctly, the access device transmits a negative acknowledgement (NACK) in order to request a retransmission of the erroneously received packet.
In addition to monitoring the PHICH for an ACK/NACK, in the case of an uplink transmission, the UA monitors the PDCCH for a DCI message including information indicating whether or not a packet retransmission (i.e., that a packet was incorrectly received) or a new transmission (i.e., that a packet was correctly received) should occur and for a resource grant for a new transmission or a re-transmission. Where a data packet has to be retransmitted, if radio conditions have changed, an access device may identify a more optimal resource for packet retransmission and transmit a new DCI message indicating the new grant to a UA. The process of changing a resource grant for retransmission is referred to generally as adaptive retransmission. If a new resource grant is not received for retransmission, the UA simply uses the previously granted resource to retransmit.
LTE uplink transmissions are divided into eight separate 1 millisecond sub-frames. DCI messages and ACK/NACK messages are synchronized with uplink sub-frames so that they can be associated therewith implicitly as opposed to explicitly, which reduces control and HARQ overhead requirements. For instance, in LTE systems, a DCI message is associated with a sub-frame four milliseconds later so that, for example, when a DCI message is received at a first time, the UA is programmed to use the resource grant indicated therein to transmit a data packet in the sub-frame four milliseconds after the first time. As another instance, in an LTE system an ACK/NACK is associated with a sub-frame four milliseconds prior so that, for example, when an ACK/NACK is received at a first time, the UA is programmed to associate the ACK/NACK with the data packet transmitted in the sub-frame four milliseconds prior to the first time.
In many cases it is desirable for an access device to transmit a large amount of data to a UA in a short amount of time. For instance, a series of pictures may have to be transmitted to an access device over a short amount of time. As another instance, a UA may run several applications that all have to transmit data packets to an access device essentially simultaneously so that the combined data transfer is extremely large.
In the case of uplink transmissions, a separate HARQ process is maintained for each uplink sub-frame used. In the case of LTE, the access device maintains a HARQ process buffer for each of the eight uplink sub-frames and packets are retransmitted by a UA in the same sub-frame as an original packet transmission. After a packet is correctly received by an access device, a new packet uplink transmission is initiated by the access device transmitting a DCI message including an NDI to the UA. Between NDIs, the access device combines transmissions occurring in the same sub-frame. Thus, an access device can associate retransmitted packets with prior transmitted packets by simply using sub-frame numbers.
One way to increase the rate of data transmission is to use multiple carriers (i.e., multiple frequencies) to communicate between an access device and UAs. For example, a system may support five different carriers (i.e. frequencies) and eight sub-frames so that five separate eight sub-frame uplink transmission streams can be generated in parallel. In multi-carrier systems, a separate HARQ process is maintained for each sub-frame/carrier combination. For instance, in a five carrier system with eight sub-frame communications, forty different HARQ processes are maintained.
While adaptive retransmission in a single carrier system can be supported by current DCI message formats, unfortunately, in multi-carrier systems where a UA transmits packets using multiple carriers in a single sub-frame (e.g., first and second carriers may be used to simultaneously transmit first and second separate packets in sub-frame 7), currently there is no way for an access device to control UA cross carrier adaptive retransmission. For example, where first and second carriers are simultaneously used to transmit different first and second packets in sub-frame 7 to an access device and it would be optimal to retransmit the first packet using a fourth carrier, there is no way, using current DCI messaging formats, to distinguish the seventh sub-frame associated with the first carrier from the seventh sub-frame associated with the second carrier.